Ag3 PCA example notebook

[alimanfoo]

I've added a new example notebook to the Ag3 user guide:

• Investigating population structure with PCA

This shows how to set up and run principal components analyses, with some examples analysing samples from central and eastern Africa.

It can be run via Google Colab but requires scanning reasonably large amounts of data to extract genotypes for input to PCA, and so can take a little while to run, up to 20 minutes depending on how many samples are being included. But hopefully a useful resource.